Morphological variation of rice bulliform phytoliths in soils under wild and domesticated rice in China

The issue of finding reliable indicators that can distinguish domesticated from wild rice in archaeological sediments remains problematic, particularly when applying phytolith morphometrics in mixed soil contexts. Here we develop a modern paddy soil reference framework to resolve long-standing controversies regarding rice bulliform phytolith criteria.

Morphometric analysis was conducted on 5800 rice bulliform phytoliths from 116 modern topsoil samples across tropical/subtropical South China, supplemented by ancient rice phytoliths before rice domestication extracted from archaeological sediments.

Size parameters (vertical/horizontal length) exhibited no statistically significant differences between wild (Oryza rufipogon, Oryza officinalis) and domesticated rice (Oryza sativa) in soil assemblages, confirming their inadequacy as domestication indicators. In contrast, the bulliform body/handle length ratio (b/a) provided a potentially diagnostic criterion: wild populations exhibited significantly lower mean ratios (0.68 ± 0.11) than domesticated systems (0.94 ± 0.08).

We propose a statistical range of b/a for identifying domesticated rice in early Neolithic archaeological sediments prior to the emergence of Oryza indica. We further demonstrate that the ≥ 40% threshold for bulliform phytoliths with ≥ 9 fish-scale facets reliably distinguishes domesticated from wild rice in soil assemblages, regardless of morphological variation in rice bulliform phytoliths. The b/a parameter resolves limitations of size-based methods and offers a complementary morphometric indicator for tracing rice domestication, particularly integrated with fish-scale facet analysis.